The best source for me at the very moment is
Harald-Helmut Vogt: Messerschmitt Bf 109 Einsatzmaschinen - Das Nachschlagewerk
http://www.amazon.de/Messerschmitt-B.../dp/B007RKQ492
There´s an excellent chapter about the pressurized G-variants with drawings, photographs etc. The head armour, in fact, is a vertical plate of steel closing the backpart of the moving hood including two triangular glazed cutouts to provide rear view for the pilot. The lower end of the steel plate seals the pilot´s compartment from the not-pressurized rear compartment of the cockpit.
Unfortunately I dont have the book here with me but hte armour feature is so characteristic it can be recognized on several photos.
Of course, desiccant tablets and the missing "Kiemenspalten" vents at the fuselage sides are an important feature, too.
Concerning the "Kiemenspalten" on not-pressurized variants I´m not quite sure but I think they´re only placed at the port side of the cockpit?
Regards
Roland